Prevent calculation on null childNode during resize observation

[thompsongl]

Summary

elastic/kibana#34271 raises the case where EuiResizeObserver using the fallback MutationObserver API results in errors when the observed element is removed.

Two possible scenarios where this.childNode ref becomes null but onResize can still occur:

1. prior to the initialization requestAnimationFrame
2. prior to observer.disconnect() but onResize is already queued

Changes address the two above by calling disconnect before changing the childNode ref, and preventing calculations when childNode is null

Checklist

- [ ] This was checked in mobile
- [ ] This was checked in IE11
- [ ] This was checked in dark mode
- [ ] Any props added have proper autodocs
- [ ] Documentation examples were added

• A changelog entry exists and is marked appropriately

- [ ] This was checked for breaking changes and labeled appropriately
- [ ] Jest tests were updated or added to match the most common scenarios
- [ ] This was checked against keyboard-only and screenreader scenarios
- [ ] This required updates to Framer X components

[chandlerprall]

Changes LGTM; needs a changelog